This invention relates to a primer coating for use on polymeric substrates, and more particularly, to a primer coating for enhancing the adhesion of liquid toner to polymeric films which are designed to be printed using an electrophotographic printing device.
In recent years, the use of on-demand digital printing machines utilizing liquid electrophotographic systems has become widespread. In such systems, liquid toner images are formed on a photosensitive drum and then electrically transferred onto an intermediate transfer blanket or belt for printing on a paper or polymeric film sheet or web. An example of a liquid toner for use in electrostatic printers is described in U.S. Pat. No. 5,407,771, and printers using such liquid toner are commercially available from Hewlett-Packard Company under the trade name HP Indigo. Polymeric films are often preferred for use over paper in electrostatic imaging because they are moisture resistant, flexible, and can be clear or opaque. However, liquid toners do not transfer well and/or adhere well to such films or paper substrates unless they have been treated with a coating or primer (also referred to as “optimized”) to enhanced the adhesion of liquid toners. A number of coatings or primers have been developed for use on substrates such as polymeric films which render the surface of the films more receptive to liquid toners. For example, a polyamide resin in solution is known for use on such polymeric films. However, because the polyamide is dissolved in a solvent, this can lead to environmental problems when the solvent is evaporated. In addition, the solution is difficult to handle at low temperatures, as the polyamide tends to lose solubility. The coating also has a limited shelf life in storage prior to printing.
Other coatings have been developed based on ethylene-acrylic acid copolymers. As described in U.S. Pat. No. 5,789,123, an ethylene-acrylic acid copolymer based coating is used to enhance the adhesion of liquid toner to polymeric substrates such as biaxially oriented polypropylene, polyethylene terephthalate and polyvinyl chloride. However, the ethylene-acrylic acid copolymer based coating must be used in conjunction with an additional precoating which is required to form the bond between the coating and polymeric substrate.
Accordingly, there is still a need in the art for a primer coating which provides good liquid toner adhesion to a variety of polymeric substrates without the need for the application of additional precoatings, is stable and easy to handle over a wide range of temperatures, and which has an extended shelf life.